In the case of measuring tubes for magneto-inductive flow measuring devices, measuring tubes of plastic are known but, for the most part, measuring tubes with a support tube of metal, especially of steel, are used. In the latter case, however, there is the problem that the material of the measuring tube is conductive. In order to enable a voltage to be tapped on the measuring electrodes, the measuring tube must be electrically insulated. For this, usually an insulating plastic layer, a so called liner, is used. The adhesion of such materials to the metal surface is, however, not optimal. Additionally, the liner can under process conditions be mechanically deformed.
Therefore, a perforated sheet is inserted, which serves the liner material as reinforcement means. This reinforcement means is inserted into the support tube and reinforces the liner material. German patent, DE 10 2008 054 961 A1 of the applicant discloses a number of possible variants of such a perforated sheet, for example, a sheet which has only a segment-wise perforating.
A major topic is the anchoring of the liner in the support tube. An anchoring opportunity is described in the aforementioned DE 10 2008 054 961. In such case, the reinforcement means is fixed to the measuring tube by a press-fit, so that the reinforcement means is fixed in the support tube only by compression, however, without a material bonded connection.
This variant has basically proved itself as practical and cost effective and can be done with little manufacturing effort. However, over longer periods of time, the compressive stress of the perforated sheet can lessen and the liner with the perforated sheet can then turn in the measuring tube.
Another opportunity for anchoring is described in German patent, DE 10 2006 018 415 A1. In such case, of concern are two reinforcement means manufactured as half shells, which can be manufactured as perforated sheets. These are secured terminally to the measuring tube with material bonded connection via welding tabs.
The welding of a perforated sheet has the disadvantage that, most often, only little space is left between the perforated sheet and the support tube for the casting of the liner material behind. In the case of temperature differences between perforated sheet and support tube, thermal expansion differences can lead to loosening of the weld locations over longer periods of operation.
A lattice for reinforcement of the liner is additionally disclosed in U.S. Pat. No. 5,773,723 A1 and US 2008/0196510 A1 likewise embodied as a reinforcement means in the form of a perforated sheet or lattice in a support tube. The function of reinforcement is also described therein. As can be seen from the figures, these reinforcement means have, however, no anchoring to the support tube but, instead, are embedded in the liner without anchoring.
In the case of rubber liners, PFA or PTFE liners, these form as is known only extremely weak adhesive bonds to the carrier measuring tube. An undesired twisting of the liner in the measuring tube in the case of longer use is, consequently, probable, the more so since no additional anchorings of the liner are provided.
An alternative opportunity of support for the perforated sheet is described in European patent, EP 1 039 269 A1. In such case, a sintered material is distributed continuously in the support tube and forms a tubular reinforcement means, which extends almost completely over the inner surface of the support tube. The material of the lining can penetrate and anchor in the pores of the loose, granular, respectively foam like, sintered material. The in-situ sintering provides an especially good bonding of the supporting sintered material to the steel support tube.
The sintered material is, however, comparatively expensive and additionally complexly implementable in the production process.